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spark counter[′spärk ‚kau̇nt·ər]
an instrument that records charged particles; it is based on the generation of spark discharge in a gas as a result of the impact of a charged particle. The counter supplies data on the passage of a particle in the form of an electric pulse and a bright spark appearing near the path of the particle. The spark is accompanied by a shock wave and a clearly audible sound.
A spark counter consists of two plane-parallel electrodes mounted in a hermetically sealed volume filled with an inert gas (argon) and vapors of organic substances (alcohol, ether, and so on) at a total pressure of 0.05–2.0 meganewtons per sq m (MN/m2), or 05.-20.0 atmospheres (atm). A constant voltage (several kilovolts) is applied to the electrodes. Electrons that form along the path of the particle through the gas as a result of ionization of atoms of gas are accelerated by the field, ionize atoms of gas (impact ionization), and thus create electron-photon avalanches, which develop into spark breakdown between the electrodes.
In contrast to a Geiger-Muller counter, in which the field is nonuniform and the electrons that have formed drift slowly into the region of a strong electric field and produce impact ionization only in the vicinity of the filament, the field in a spark counter is uniform, and collision ionization can begin at any point in the interelectrode space. This results in a very short time lag of the discharge with respect to the time of passage of the particle. Delay times of 100 to 10 nanoseconds have been produced in spark counters with a gap of 0.1–0.2 mm and a pressure of 0.3–2.0 MN/m2, or 3–20 atm). This makes possible the use of spark counters to measure very short time intervals, such as particle decay times. However, spark counters have a dead time (the recovery time after breakdown) of about 1 millisec; thus, they cannot be used for dense particle streams.
Spark counters with plane-parallel electrodes described above are the forerunners of spark chambers. In addition to this design, spark counters for alpha particles also exist. In this type of counter a metal plate is the cathode, and the anode in the form of a metal filament is stretched on insulators parallel to the cathode at a distance of 1.5–2.0 mm. The counter is usually operated in air and at atmospheric pressure. Upon irradiation of the gas-discharge gap of the spark counter by beta particles or gamma quanta, no effects are observed, since the ionizing power of electrons is low. However, an alpha particle has a much higher ionizing power; if such a particle passes between the electrodes, the nature of the discharge changes instantaneously, and a spark jumps across in the path of flight of the alpha particle. Therefore, spark counters of this type can be used for recording alpha particles in the presence of intensive beta and gamma radiation.
The pulse rise time is short (about 100 millisec); total pulse duration usually is not less than 10 ~4 sec.
M. I. DAION